Explanation

The “only” that follows the blank indicates that the effect of the compound in water is an exception to its general state in the air. Since the compound is dangerous underwater, it follows that it is generally harmless in the air. The blank is preceded by “not,” so in order for the phrase to mean “harmless,” it must be filled by words that are synonymous with “harmful.” “Deleterious” and “detrimental” mean just that, and as synonyms they produce sentences alike in meaning, so they are the correct response. Of the other choices, “trivial” and “inconsequential” are synonyms, but they, like the other two choices, do not produce the necessary contrast with the last part of the sentence. Thus the correct answer is deleterious (Choice C) and detrimental (Choice E).
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The trigger words are right after the semicolon. "only in the form of"- these suggest that the first part of the sentence should anything but damaging. Since there is a negative word already placed in front of the blank, to get something to mean the opposite of damaging, we have to put something that indicates damage in some form, because of that 'not'. So, C and E are the right choices for this blank.

Regards,
Scrat

why not F?

Because has no sense in the economy of the sentence. really, it has no sense at all.

You need a word/s that show the substance is harmful unless in a certain scenario.

Hope this helps

Regards
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